x86: move IDT away from address 0

Address 0 is also used for the SIPI vector (which is probably something worth
changing as well), and now that we call setup_idt very early the SIPI vector
overwrites the first few bytes of the IDT, and in particular the #DE handler.

Fix this for both 32-bit and 64-bit, even though the different form of the
descriptors meant that only 32-bit showed a failure.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>